The invention relates to the field of photo-bioreactors, i.e. to devices that allow photosynthetic microorganisms to grow in controlled manner.
The invention relates more precisely to a method of improving the rate of growth of phototropic micro-organisms in a culture in suspension in a liquid reaction medium, in which method the culture is subjected to a photosynthetic reaction activated by radiation passing through a transparent wall of a chamber of thickness d in a photobioreactor in which the reaction medium circulates.
It is known that the performance of systems for producing phototropic microorganisms vary depending on the species being cultured, and for a given species, they vary with concentration and with the thickness of the layer of culture that is subjected to radiation. Inter-specific variations are due to the fact that certain species suffer more than others in excess light, have better capacity to use low levels of light, and/or can be cultured at higher concentrations than others.
Variations in intraspecific performance are essentially associated with the thickness of the layer of culture subjected to radiation in the chamber and to the concentration of the culture. It has been observed that the absorbance or optical density of a microorganism in culture is more or less conserved regardless of the thickness d of the layer and the concentration c of the culture. This amounts to saying that dxc3x97c is equal to a constant expressed in terms of grams of microorganism dry weight per square meter of photosynthetic area, also known as density per unit area. The value of this constant lies in the range 10 grams per square meter (g/m2) to 40 g/m2 depending on species.
It can thus be seen that the average thickness d of chambers or pipes in which the photon reaction takes place ought to change with the species of microorganism being cultured for any desired concentration of the culture.
For a given species, and high concentration, the thickness d must be small, so as to make light accessible to the entire population of microorganisms in suspension in the liquid reaction medium circulating in the reactor chamber, in order to improve the yield of the reaction. However, too small a thickness d can give rise to serious head losses in the circuit.
It is always a good plan to increase the final concentration of microorganisms in the liquid reaction medium so as to limit the volumes of liquid that need to be processed upstream (preparing the nutritive medium) and downstream (concentrating biomass, treating the clarified medium before rejecting it). In order to improve the performance of bioreactors, it must therefore be possible to modify the thickness d of the chambers of the reactor as a function of the species being cultured. Unfortunately, this is not easily achieved in practice.
The object of the invention is to propose a method that is easy to implement and that makes it possible to adapt a given photobioreactor of the continuously operating recirculation type as a function of the micro-organism species being cultured and as a function of the concentration desired for the culture, regardless of the thickness d of the reactor chambers, so as to improve the rate of growth of the microorganisms.
The invention achieves this object by the fact that in the proposed method the absorbance of the culture is adjusted by adding transparent or reflecting particles to the reaction medium, which particles are of a density that is substantially equal to that of said reaction medium, the volume percentage occupied by said particles in the reaction medium being a function of the species of microorganism being cultured, of the thickness d of the chamber, and of the desired final concentration for the culture in the reaction medium.
The particles accompany the reaction medium in uniform manner in the flow. They have the effect of reducing optical path lengths within the chambers. Increasing the percentage per unit volume of particles in the reaction medium amounts to decreasing the thickness d of the layer in the chamber without increasing head losses, or to diluting the culture without adding water, where adding water would be undesirable from the point of view of collecting biomass.
Another expected effect of particles is that they will clean the optical walls of the reactor, and they will do so regardless of its shape.
The quantity of particles is a function in particular of the desired final concentration of biomass that is to be obtained.
When a bioreactor operates continuously, this concentration remains constant. However, when starting a new culture and bringing the system up to full load, concentration is well below the desired final concentration. The microorganisms are then subjected to radiation at intensity that is too high, and that can harm yield.
According to another characteristic of the method of the invention, the turbidity of the reaction medium is measured and the intensity of the radiation is adjusted as a function of the measured turbidity.
Advantageously, the particles are compact in shape so as to avoid effects of lift and of superposition. The diameters of the circles escribed around the particles preferably lie in the range 0.5 millimeters (mm) to 10 mm. By way of example, particles are used that are made by coating hollow microbeads of glass in a transparent plastics material in proportions such that the density of the resulting composite is equal to that of the reaction medium.
The invention also relates to a photobioreactor of the continuously operating recirculation type for implementing the method of the invention.
The photobioreactor comprises: a closed loop in which it is possible to circulate a culture in suspension in a liquid reaction medium; at least one chamber of thickness d provided in said loop and defined by a transparent wall; means for emitting radiation through said transparent wall; means for introducing particles into said loop; and means for ensuring that said particles are retained within said loop.